GeoModeller uses a Top VS Bottom convention to determine the inside and outside volumes of a unit bounded by its geological interface. 

As geological interfaces are interpolated from discrete interface points, these may be either mapped as:

• the top of the formation that it is attributed to
• the bottom of the formation that it is attributed to

An immediate consequence of this distinction is that a "fill-up" formation is added either at the highest or lowest position on the stratigraphic pile where data for formation is missing by definition:

• top referenced models have a default DefaultCover formation
  
• bottom referenced models have a DefaultBasement formation
  

The Top VS Bottom distinction is set a the scale of the whole project, therefore either all interface points for all formations in all series map for tops or bottoms. It is not possible to work with a mix of both conventions.

When switching from one convention to the other, the same list of interface points mapped for the top of a basal unit will now correspond to its bottom. 

The volume fill direction is reversed when changing references. This will bring drastic change the geometry of the units and existing data may have to be reattributed to other units to maintain consistency across the model.

Below is an example of how much a reference switch alone may affect a vertical section.

The original model features a basin and an intrusion using Bottom reference with consistent structural data.

Reference switched version of the same model with no other changes, the intrusion is found at the top of the model as it is now modeled "inside out" and replaces the expected DefaultCover formation due to its Erode stratigraphic relationship.